The present invention relates to a lead frame device, in particular a lead frame device for a voltage-sensitive micromechanical component, as well as a corresponding manufacturing process.
Although applicable to any microelectronic and micromechanical components, the present invention and the general problem underlying it are described with reference to a micromechanical pressure sensor semiconductor chip in silicon technology.
In general, it is necessary to pack microelectronic or micromechanical components (ICs, chips, sensors, etc.), which exists initially in the form of chips or dies, in order to protect them from environmental influences. In this context, the problem of contacting the chips to the external supply lines and to the contiguous hermetic packing must be solved.
A customary process includes the following steps: mounting the chips on a lead frame strip, forming bonds between the external leads or terminals of the lead frame and the bonding pads or bonding spots of the chips, deforming using plastic material, and separating the deformed components.
A customary lead frame strip is depicted in a cutaway view in FIG. 1. In FIG. 1, 10 generally designates the lead frame strip, 15 a die pad or a chip mounting surface, 20 a multiplicity of external leads provided, and 25 tie bars as mounting support for die pad 15.
On a lead frame strip 10 of this type, which is usually made from a stamped or etched copper strip, i.e., of a uniform material, a multiplicity of chips is simultaneously mounted by an automatic device onto die pads 15, e.g., using cementing, soldering, etc., bonded, packed, and finally separated.
The problem underlying the present invention generally lies in the fact that due to the varying thermal coefficients of expansion of the lead frame material, the mounting material (e.g., cement or solder), and the chip material, mechanical stresses can be generated. These stresses can have drastic consequences for the micromechanical pressure sensor semiconductor chip in question, because they can be incorrectly interpreted in routine measurements as external pressure.
In general, the possibility exists to manufacture the lead frame strip, instead of out of copper, out of another, thermally more adapted material, such as FeNi42%. Then the entire lead frame strip would be composed of this material, which would lead to problems in the further automatic assembly. In addition, it can be necessary to simultaneously mount different chips having different thermal requirements. In this case, the above solution could only yield an optimal adaptation for one of these many chips.
The lead frame device according to the present invention and the corresponding manufacturing process have the advantage that they offer a simultaneous optimal adaptation for a multiplicity of different components having different thermal requirements. The stresses exerted by the different thermal expansion of lead frame and chip or chips are minimized. However, on the other hand, the material properties are maintained that are well suited for the further automatic processing of the lead frame strip, such as good thermal conductivity and good bonding capacity (in particular of the leads). Only the die pad is supplemented or replaced with respect to its material. Thus no substantial alteration of the production line is required.
The idea underlying the present invention lies in the fact that in the die pad area of the lead frame strip, i.e. the mounting area of the die pad, which either has a die pad made of the first material or which has no die pad made of the first material, a die pad is mounted that is adjusted to the chip to be mounted, namely in a supplemental working step, which proceeds analogously to the step of the actual chip assembly but is upstream of it. In this way, materials having varying thermal coefficients of expansion can be integrated in one lead frame strip. In other words, as a result of the present invention, a plurality of different chips made of different materials can be accommodated in one housing on one lead frame.
According to one preferred refinement, the first material is copper or a copper alloy or a copper compound, and the second material is a material that is thermally adapted to the material of the chip, preferably FeNi42% or Kovar. Thermally adapted, in this context, means essentially an adaptation to the thermal coefficients of expansion. However, there can be cases in which thermal conductivity or heat-absorption capacity can also play a role.
According to a further preferred refinement, the die pad made of the second material is mounted on a die pad made of the first material, which is a part of the lead frame. As a result, no modification of the manufacturing process for the lead frame strip is required, but rather only an additional mounting step for the die pad made of the second material.
According to a further preferred refinement, the die pad made of the second material is mounted at its periphery on a die pad frame made of the first material, which is a part of the lead frame. Thus, in the manufacturing process for the lead frame strip, a part of the die pad, originally provided, made of the first material, is stamped out or etched. This configuration is advantageous for the purpose of minimizing positional tolerances.
According to a further preferred refinement, the die pad made of the second material is mounted on tie bars made of the first material, which are parts of the lead frame. Thus, in the manufacturing process for the lead frame strip, the entire die pad, originally provided, made of the first material, is stamped out or etched.
According to a further preferred refinement, the die pad made of the second material is mounted on the lead frame using caulking, cementing, soldering, or welding. These are customary connecting procedures for two metals.
However, in this context, it should be mentioned that the second material can also be a plastic.
According to a further preferred refinement, a further die pad area is provided, within which a further chip is mounted, and a plurality of leads is arranged around the further die pad area; and a die pad made of a third material is mounted for accommodating the further chip in the further die pad area on the lead frame, the third material being either the same as the first or the same as the second or a material that is different from them. In this manner, a multichip module having optimal thermal adaptation can be produced.